Have you tried this link? http://heroes.ag.ru/heroes5/mods/zip/Maps_ToE_UnlockAllHeroes.zip I probably won't be able to help with the creation, as I have a vague idea about this part of the game mechanics. My only comment: why is this needed? Some campaign heroes will simply be unmatched on certain maps (Raelag is almost everywhere; Markel on those with neutral undead; Zephyr is also quite strong; Frida with Divine Vengeance is also quite powerful). So, this could significantly spoil the balance.

Added 3 minutes later
By the way, I have an idea. After figuring out this mod, you could try to integrate it into all other maps that you will play, including randomly generated ones.